The present invention is directed to methods and apparatus for removing sludge byproducts that are created by water filtration processes from settling basins and tanks commonly found in water and wastewater treatment plants.
The settling basins in water and wastewater treatment plants are typically circular or rectangular concrete structures that have sloped bottoms that collect and store sludge. The basins are periodically drained and the sludge that has accumulated is flushed out through drains in the basin floor. Removal of the accumulated sludge is an important part of the overall treatment process, particularly since anaerobic bacterial activity may develop in the collected sludge over time. Therefore, in an ideal situation, the sludge is drained or removed without disturbing the material through excavation or manual removal. Moreover, it is further desirable from a cost and efficiency standpoint to be able to effectively remove the sludge from the basin when accumulation requires its removal.
Methods of removing sludge on a more or less continuous basis have been developed in an effort to reduce the need to completely drain and flush the basin. Techniques such as header and lateral piping systems, scraper mechanisms and vacuum or suction removal systems have been tried, without complete success.
Sludge removal systems are known where a moving apparatus is directed across the floor of the basin to remove the sludge in its path. The unit can be guided by rails and may use static head or eduction to remove the sludge. In some variations, the sludge collection basin is cone-shaped and the sludge collection apparatus moves along a circular or spiral path over the surface of the cone. Such systems are cumbersome and expensive and require a complex system of moving parts and precision machinery prone to breakdown in the gritty environment of a sludge bed.
In other systems, the inherent problems of moving equipment are avoided by providing a series of fixed pipes to remove the sludge. The pipes are selected so that a number of perforated pipes of small diameter are connected to larger diameter pipes, which are in turn connected to a lesser number of larger diameter pipes, and so on, until ultimately, the piping xe2x80x9ctreexe2x80x9d meets at a single header pipe of relatively large diameter that is controlled by a single collection valve. The single valve is opened to flush sludge into drains by means of the static head of water in the basin. In order for fixed grid sludge collection systems to collect evenly, it is critical that the flow capacity of the grid be compatible with the flow capacity of the downstream piping including the header, valve and basin outlet. Therefore, it is necessary to either use a very large valve and basin outlet in combination with a grid covering the entire basin or to use multiple valves and basin outlets in combination with smaller grids.
Another limitation to fixed sludge collection systems is that sludge is often not efficiently removed because the water in the basin tends to flow around the sludge and into the collection system. Typically, sludge may flow into the collection system when the valve first opens, creating a hole or depression in the sludge. This depression is known as a xe2x80x9crat hole.xe2x80x9d Once this depression exposes the collection orifice, water enters the collection system rather than sludge. Fixed collection systems that have valves open for extended periods of time usually collect more water than sludge. The tendency toward xe2x80x9crat holingxe2x80x9d is dependent upon certain characteristics of the sludge such as its composition, concentration, viscosity, and compressibility. Sludge found in water treatment processes may have different compositions, varying amounts of suspended solids, and therefore different characteristics. For example, alum sludge will have different characteristics than ferric sludge or lime sludge.
A limitation to systems using single large outlets is that the system must be operated to remove sludge from the entire basin as soon as one area exhibits significant sludge accumulation. This type of operation is inherently inefficient and either wastes water unnecessarily or allows sludge to accumulate more heavily than is optimal for the process. A limitation to systems using the multiple basin outlets is the expense of the outlets and the difficulty in retrofitting the basin.
Finally, sludge removal is also sometimes attempted by providing systems that float and skim the sludge from the water overlying the bottom of the basin. However, such systems are inefficient, expensive and require complex systems of piping and suction, the latter of which are prone to breakdown since a gritty slurry of sludge and water must be skimmed and pumped.
Additionally, all of the known systems discussed above are difficult if not impossible to retrofit into existing sludge collection basins.
Therefore, it would be desirable to provide a sludge collection, system that is simple and reliable, while still effectively removing sludge. Additionally, it would be desirable to permit the amount of removal applied to various areas of the bed to be adjusted or varied based upon sludge accumulation patterns. Finally, such a system would ideally be adaptable for both new construction and retrofit applications.
The present invention provides a sludge collection system that is comprised of a manifold having a plurality of manifold inlets and a manifold outlet, a plurality of collection laterals, for collecting sludge, that are in fluid communication with the manifold by way of the manifold inlets, a drain, in fluid communication with the manifold outlet for removing sludge from the manifold, and one or more collection valves that selectively permit flow of sludge from the collection laterals through the manifold inlets and through the manifold.
The specialized piping of the sludge collection system of the present invention provides improved hydraulic characteristics. One advantage of the sludge collection system disclosed is that the performance of the system can be regulated and adjusted to meet the requirements of the filtration system. Another advantage of this invention is the ability to effectively remove sludge through smaller or fewer basin outlets than conventional fixed grid designs. The disclosed system is thereby more cost effective for retrofitting into existing basins.
The present invention uses the static head of the water in the basin to push sludge and water into a sludge collection grid, through a valve, through main drain piping and out of the basin. Each sludge collection basin is divided into a number of collection zones, and each zone may be sized and independently operated to allow the hydraulic characteristics of the system to be optimized. Optimization is preferably obtained by matching the flow rates of each zone to the capacity of the piping system used to remove the sludge from the basin.
The flow rate from each zone is dependent on the available static head and the diameter and number of the orifices in the zone. The flow rate can be optimized by varying the diameter and/or the number of orifices in the collection grid. In addition, the laterals may be spaced at varying pitches to provide further flexibility. The zones allow the system to provide the required amount of removal capacity in each area of the basin. In most basins, sludge tends to collect more rapidly in one area, and the removal capacity of each zone is thus preferably tailored to the anticipated sludge accumulation rate. The spacing of the orifices and the laterals can also be optimized for the collection of various types of sludge in order to reduce the effect of xe2x80x9crat holing.xe2x80x9d As a result, the present invention is more cost effective and efficient than previous systems.
In a preferred embodiment, the sludge collection system of the present invention comprises one or more header and lateral piping systems installed on the bottom of the basin that together create a sludge grid disposed within a zone. The header piping from each zone is connected to a valve and the valve selectively allows water and sludge from one or more selected sections to flow into a main drain pipe, and out of the basin for further processing. For example, the collected sludge can then be transferred to a holding tank or clarifier. The sludge may then be dewatered by one of several available methods and the supernatant returned to the head of the plant or otherwise carried away.
In various preferred embodiments, the sludge collection system of the present invention can be provided with either automated or manual controls. These controls preferably allow the operator to control the frequency and the duration of the operation of the valves within each zone independent of the other zones. In this manner, sludge can be removed from a particular zone or zones as necessary. The optimal frequency of withdrawal is dependent on site specific factors such as the condition of the raw water, chemical feeds, sludge characteristics, and the plant""s capacity to handle concentrated sludge and unfinished water.